High strength aluminum alloy

ABSTRACT

1. A HIGH STRENGTH ALUMINUM CASTING ALLOY HAVING A TENSILE STRENGTH OF NOT LESS THAN 50 KG./MM2 AT ROOM TEMPERATURE AND AN ELONGATION OF NOT LESS THAN 1.5%, THE ALLOY CONSISTING ESSENTIALLY OF 3.0-6.0% BY WEIGHT ZINC, 2.0-4.0% BY WEIGHT CHROMIUM, 0.02-0.2% BY TITANIUM, 0.1-0.4% BY WEIGHT CHROMIUM, 0.02-0.2% BY WEIGHT LITHIUM, AND THE BALANCE ALUMINUM AND INEVITABLE IMPURITIES.

United States Patent 3,843,357 HIGH STRENGTH ALUMINUM ALLOY Itaru Niimi, Nagoya, Yasuhisa Kaueko, Toyota, Yoshlro Komiyama, Okazaki, and Masaoki HHShlIlIOtO Toyota Japan, assignors to Toyota Jidosha Kogyo lhabushiki Kaisha, Toyota-elm, Japan No Drawing. Filed Aug. 29, 1973, Ser. No. 392,480 Claims priority, application Japan, Oct. 31, 1972, 47/ 109.098 Int. Cl. C22c 21/00 US. Cl. 75146 2 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to high strength aluminum alloy for casting having a tensile strength not less than 50 kg/mm. at high temperatures and an elongation not less than 1.5%. Essentially the alloy consists of 3.0- 6.0% zinc, 2.0-4.0% magnesium, 01-04% titamum, 0.1-0.4% chromium, 0.020.2% lithium and the balance aluminum and inevitable impurities. Optionally, copper may be added but not more than 2.0%.

BACKGROUND OF THE INVENTION Heretofore, aluminum alloy castings specified in the 118 (Japanese Industrial Standard) have been mainly used. Alloys such as aluminum-copper, aluminum-silicon and aluminum-magnesium commonly have a tensile strength not more than 35 'kg./mm. With the increasing demand for aluminum alloys in recent years, aluminum alloys having higher performance are required.

Particularly, for the purpose of weight reduction, aluminum alloy castings having a high tensile strength at room as well as high temperatures are being increasingly required. These alloys are replacing iron materials such as iron castings. According to this requirement, aluminumzinc-magnesium alloys have been developed, but they must contain a very large amount of zinc or magnesium in order to attain a tensile strength not less than 50 kg./mrn. 0n the other hand, the alloys containing a large amount of zinc or magnesium are subject to hot cracking during casting or reduction of the stress corrosion resistance. Obviously, such defects render these materials unacceptable.

SUMMARY OF THE INVENTION The high strength aluminum alloy of the present invention is used for casting purposes and is directed to and improving the strength of the casting during heating. Hot cracking is prevented, and the weight of casting is reduced.

The high strength aluminum casting alloy of the present invention is based on an aluminum-zinc-magnesium alloy to which various elements are added with their amounts being limited as follows. The high strength aluminum casting alloy of the present invention is composed of, by weight, 3.0-6.0% zinc, 2.0-4.0% magnesium, 0.l-0.4% titanium, (Ll-0.4% chromium, 0.02-0.2% lithium, copper not more than 2.0%, and the remainder of aluminum and inevitable impurities.

The difference between conventional aluminum alloys and the high strength aluminum casting alloy of the present invention is best described by referring to experimental results. In the experiments, an aluminum-zincmagnesium or aluminum zinc-magnesium-copper alloy was used as the conventional aluminum alloy and an aluminum-zinc-magnesium-lithium or aluminum-zincmagnesium-lithium-copper alloy as the high strength aluminum casting alloy of the present invention.

The alloy compositions are indicated in Table 1.

Further, the high strength aluminum casting alloy of the present invention was not limited to those described and the series of numbers used in the composition of this alloy are referred to in the following experimental examples.

TABLE 1.COMPOSITION 0F ALLOY Number Alloy type Zn Mg Cu Li Ti Cr 1. Conventiona1 5.5 3.0 0.1 0.2 2 Invention 5. 5 3. 5 0. 12 0. 1 0. 2 3. Conventional 5. 5 3. 5 1. 0 0. 1 0. 2 4 Invention 5. 5 3. 5 1. 0 0. 12 0. 1 0. 2 5. Conventiona 4. 0 3.0 1.0 0.1 0.2 6 Invention 4. 0 3.0 1.0 0.08 0.1 0.2 7 Conventional. 3. 5 2. 0 1. 5 0. 1 0. 2 8. Invention 3. 5 2.0 1. 5 0. 05 O. 1 0. 2

The above aluminum alloys with Nos. 1 to 8 were retained, after casting, at 470 C. for 16 hours, immediately followed by quenching (in water at room temperature), and then subjected to an artificial aging at C. for 24 hours, corresponding to H8 T -Temper, and their room temperature tensile strength, high temperature tensile strength and elongation were determined. For the measurements, the T -tempered materials were machined into test pieces having a parallel portion of 8 x 40 mm.

In the high temperature tension test, the test pieces were preheated at the respective temperatures of C., 250 C., and 350 C. for 100 hours, and thereafter the pieces were set to a test machine and retained therein at the respective temperatures for 15 additional minutes before the tension test.

The test results at room temperature of the Nos. 1 to 8 aluminum alloys are indicated in Table 2.

TABLE 2.TENSION TEST RESULTS A ROOM TEMPERATURE T Tensile strength Elongation (kg/mm?) (percent) Alloy number:

1 46. l 1. 5 52. 7 2. 2 49. 0 0. 9 57. 0 1. 6 45. 5 l. 4 52. 0 2. 8 43. 3 1. 7 51. b 3. 2

TABLE 3.-TENSILE STRENGTH TEST RESULTS AT HIGH TEMPERATURE Ten ile strength (kg/mm?) at hlgh temperature of- 150 C. 250 C. 350 C.

In the above test, the test pieces were preheated at the respective temperatures of 150 C., 250 C., and 350 C. for 100 hours prior to the measurement of tensile strength at the respective temperatures.

As is clearly shown in Table 3, it has been found that the high strength aluminum casting alloy containing lithium is improved in tensile strength at high temperature as compared to the conventional aluminum casting alloy.

Then, by employing the annular mold test method, the high strength aluminum casting alloys of the present invention and the conventional aluminum alloys were compared with respect to the hot cracking property.

The hot crack test is generally described as follows. The material to be tested is cast into an annular metal mold and its contraction upon solidfying is restricted by a core. The total length of cracks generated thereby is determined. The results are indicated in Table 4.

TABLE 4. HOT CRACK TEST RESULT Alloy No.: Hot crack length 1 Complete fracture. 2 mm. 3 Complete fracture. 4 mm. 5 mm. 6 0.

- casting alloy.

Lastly, the reasons for limiting the numerical value of the components in the high strength aluminum alloy casting of the present invention is described below.

A content of zinc less than 3.0% does not provide a tensile strength not less than 50 kg./mm. while its content exceeding 6.0% adversely affects the castability and the stress corrosion resistance of the material.

A content of magnesium less than 2.0% does not provide a tensile strength not less than 50 kg./mm. while its content exceeding adversely affects the castability and the stress corrosion resistance of the material.

While a tensile strength of not less than kg./mm. can be obtained without copper, the tensile strength increases with the copper content. However, a copper content exceeding 2.0% adversely affects the castability and the stress corrosion resistance of the material.

A content of lithium less than 0.02% decreases the tensile strength at both room temperature and high temperatures, while a content exceeding 0.2% reduces the tensile strength at both room temperature and high temperatures. With more than 0.2% lithium elongation is reduced as well as the eifect for preventing hot cracks. Also, more than 0.2% lithium deteriorates the surface condition of casting.

A content of titanium and chromium less than 0.1% each does not produce the effect for preventing the hot crack, while their respective contents exceeding 0.4% lowers the elongation.

What is claimed is:

1. A high strength aluminum casting alloy having a tensile strength of not less than 50 kg./mm. at room temperatures and an elongation of not less than 1.5%, the alloy consisting esentially of 3.0-6.0% by weight zinc, 2.0-4.0% by weight magnesium, 0.10.4% by weight titanium, 01-04% by weight chromium, 0.02-0.2% by weight lithium, and the balance aluminum and inevitable impurities.

2. A high strength aluminum casting alloy as in claim 1 additionally including not more than 2.0% by weight copper.

References Cited UNITED STATES PATENTS 11/1970 NOWak --l46 5/1941 Nock 75141 U.S. Cl. X.R. 75-141 

1. A HIGH STRENGTH ALUMINUM CASTING ALLOY HAVING A TENSILE STRENGTH OF NOT LESS THAN 50 KG./MM2 AT ROOM TEMPERATURE AND AN ELONGATION OF NOT LESS THAN 1.5%, THE ALLOY CONSISTING ESSENTIALLY OF 3.0-6.0% BY WEIGHT ZINC, 2.0-4.0% BY WEIGHT CHROMIUM, 0.02-0.2% BY TITANIUM, 0.1-0.4% BY WEIGHT CHROMIUM, 0.02-0.2% BY WEIGHT LITHIUM, AND THE BALANCE ALUMINUM AND INEVITABLE IMPURITIES. 